1. Field of the Invention
The present invention generally relates to an optical semiconductor device, and particularly relates to a laser diode having an optical resonator having a diffraction grating.
2. Description of the Related Art
In a fast optical communication network via optical fibers, a DFB (Distributed Feedback) laser diode or a DBR (Distributed Bragg Reflector) laser diode having an optical resonator having a diffraction grating is widely used as a single-mode light source which may be modulated by fast optical modulation.
The laser diode used as a light source for a fast optical communication network is required to operate in a single-mode. Therefore, a DFB laser diode or a DBR laser diode having an optical resonator having a diffraction grating instead of a mirror is commonly used as a light source for such fast optical communication network.
FIG. 1 shows a diagram illustrating a DFB laser diode 10 of the related art.
Referring to FIG. 1, the laser diode 10 is formed on an n-type InP substrate 11. A cladding layer 12 of n-type InP, an SCH (Separate Confinement Heterostructure) layer 13 of undoped InGaAsP, and an active layer 14 of undoped InGaAs are, in turn, epitaxially grown on the InP substrate 11.
A further SCH layer 15 of undoped InGaAsP is epitaxially grown on the active layer 14. A DFB diffraction grating 15 A is formed on the SCH layer 15. Further, a cladding layer 16 of p-type InP and a contact layer 17 of P-type InP are in turn, epitaxially grown on the SCH layer 15. A p-type electrode 18 is disposed on the contact layer 17 and an n-type electrode 19 is disposed on a lower surface of the substrate 11.
With the laser diode 10 of the above-mentioned structure, the electrodes 18 and 19 serves to inject carriers into the active layer 14. Due to recombination caused by the injected carriers, an optical radiation is generated in the active layer 14. The optical radiation is guided though the SCH layers 13 and 15 and then optically amplified by stimulated emission in the active layer 14. Thereupon, an optical component tuned to an effective pitch of the diffraction grating 15A, or, having a wavelength within the range of the Bragg wavelength to the DBF wavelength xcexg of the diffraction grating 15A is repeatedly reflected by the DFB diffraction grating 15A and is selectively amplified.
However, there is a certain drawback when such a single-mode laser diode is driven by a modulation signal. Since the modulation signal alters the density of the injected carriers in the active layer 14 and thus the refractive index of the active layer, the effective period of the diffraction grating is also altered, and thus it can be said that the oscillation wavelength is altered simultaneous with the modulation signal. This effect is commonly referred to as chirping. Such chirping may cause the wavelength of an optical signal to shift from the optimum transmission band for the optical fibers, so that the transmission distance of the optical signal may be limited.
The magnitude of chirping is determined by a line-width enhancement factor xcex1, which is generally defined by an equation:                               α          =                                    ∂                              [                                  Re                  ⁢                                      {                                          χ                      ⁡                                              (                        N                        )                                                              }                                    ⁢                                      ]                    /                                          ∂                      N                                                                                                          ∂                              [                                  Im                  ⁢                                      {                                          χ                      ⁡                                              (                        N                        )                                                              }                                    ⁢                                      ]                    /                                          ∂                      N                                                                                                          ,                            (        1        )            
where "khgr"(N) is a complex susceptibility of the active layer of the laser diode, N is a carrier density, Re{"khgr"(N)} is the real part of "khgr"(N), and Im{"khgr"(N)} is the imaginary part of "khgr"(N). Re{"khgr"(N)} relates to a refractive index of the active layer and Im{"khgr"(N)} relates to an absorption of the active layer.
Given that a well-known Kramers-Kronig relationship holds between Re{"khgr"(N)} and Im{"khgr"(N)} and that Im{"khgr"(N)} is proportional to the gain g of the laser diode, the line-width enhancement factor xcex1 may also be represented by an equation:                                           α            ⁡                          (                              E                ,                N                            )                                =                                    -              P                        ⁢                                          ∫                                  -                  ∞                                ∞                            ⁢                                                                                          ∂                                              g                        ⁡                                                  (                                                                                    E                              xe2x80x2                                                        ,                            N                                                    )                                                                                      /                                          ∂                      N                                                                                                  E                      xe2x80x2                                        -                    E                                                  ⁢                                  xe2x80x83                                ⁢                                                                            ⅆ                                              E                        xe2x80x2                                                              /                                          ∂                                              g                        ⁡                                                  (                                                                                    E                              xe2x80x2                                                        ,                            N                                                    )                                                                                                      /                                      ∂                    N                                                                                      ,                            (        2        )            
where E and Exe2x80x2 represent energies and P is Cauchy""s principal value.
With a typical laser diode 10 having the active layer 14 of a bulk structure, the line-width enhancement factor xcex1 is generally of an order of 4 to 6 and therefore cannot avoid a substantial chirping effect due to the modulation signals. Whereas with a laser diode having a quantum well layer in the active layer 14 with the SCH layer 15 serving as a barrier layer, the value of the line-width enhancement factor xcex1 may be decreased to about 2. With such a quantum well laser diode, by optimizing the material and composition of the quantum well and the laser structure and by combining with the DFB optical resonator, the value of the line-width enhancement factor xcex1 may be decreased to about 1.4 to 1.8.
FIG. 2 is a graph showing a relationship between the gain and the line-width enhancement factor of the laser diode of FIG. 1. Referring to FIG. 2, it can be seen that a wavelength at maximum gain is offset from a wavelength where the line-width enhancement factor xcex1 is zero and thus the gain is negative at the wavelength where the line-width enhancement factor a is zero. Accordingly, with the quantum well laser diode of the relate art, the material and the composition of quantum wells and the pitch of the DFB diffraction grating 15 are determined such that the laser oscillates at a wavelength where the gain spectrum is positive and the line-width enhancement factor xcex1 is as close as possible to zero. However, with such a process, chirping can only be reduced to a limited extent and it is not possible to obtain sufficient gain.
Also, it is known to modify the laser diode of FIG. 1 by providing the active layer 14 of quantum dots. See, for example, Japanese laid-open patent application No. 9-326506.
FIG. 3 is a diagram illustrating a DFB laser diode 20 of the related art in which quantum dots are used as an active layer.
Referring to FIG. 3, the laser diode 20 is formed on a (001) surface of an n-type GaAs substrate 21. The laser diode 20 includes a cladding layer 22 of n-type AlGaAs having a composition of Al0.4Ga0.6As which is epitaxially grown on the substrate 21, an SCH layer 23 of undoped GaAs which is formed on the cladding layer 22 a cladding layer 24 of p-type AlGaAs having a composition of Al0.4Ga0.6As which is formed on the SCH layer 23 and a contact layer 25 of P-type GaAs formed on the cladding layer 24. Further, an active layer constituted by a plurality of quantum dots 23A is formed in the SCH layer 23 . Further, a diffraction grating 23B is formed on the SCH layer 23 in a direction of axis of the laser diode 20. A p-type electrode 26 is disposed on the contact layer 25 and an n-type electrode 27 is disposed on a lower surface of the substrate 21.
With such laser diode 20 using quantum dots, it is expected that, if the zero point of the line-width enhancement factor xcex1 is close to the peak of the gain spectrum, chirping can be effectively reduced.
FIG. 4 is a graph showing a relationship between the optical gain and the wavelength for a DFB laser diode. Again spectrum of the laser diode 20 having quantum dots has a thermal dependency of about 0.25 nm/xc2x0 C., and as can be seen from the graph of FIG. 4, the gain spectrum shifts towards longer wavelength side when there is an increase of the temperature of the laser diode. On the contrary, the Bragg wavelength of the DFB diffraction grating 23B has a thermal dependency of only about 0.1 nm/xc2x0 C. Therefore, with the quantum dot DFB laser diode 20 of the related art, there is a drawback that the change in operation temperature may cause the Bragg wavelength of the DFB diffraction grating 23B to shift out of the gain spectrum, which ceases the laser oscillation.
Accordingly, it is a general object of the present invention to provide a novel and useful optical semiconductor device which can overcome the above drawback.
It is another and more specific object of the present invention to provide a laser diode in which an absolute value of the line-width enhancement factor is minimized and the range of operating temperature is extended.
In order to achieve the above objects according to the present invention, a laser diode includes:
a substrate having a first conductive type;
a first cladding layer having a first conductive type and formed on the substrate;
an active layer including a plurality of quantum dots and formed on the first cladding layer;
a diffraction grating having a Bragg wavelength of xcexg and formed on the active layer;
a second cladding layer having a second conductive type and formed on the active layer;
a first electrode for injecting carriers having a first polarity into the active layer via the substrate; and
a second electrode for injecting carriers having a second polarity into the active layer via the second cladding layer,
wherein the diffraction grating has a pitch satisfying the following equation:
xcex94Exe2x89xa61.1xcex93
where xcex93 is the full width at half maximum (FWHM) of the gain spectrum of the active layer and xcex94E an amount of shift of an energy corresponding to the Bragg wavelength xcexg from the center wavelength energy of the gain spectrum. According to the present invention, the absolute value of the line-width enhancement factor xcex1 of the active layer can be restricted to a value less than or equal to 4.
Further, according to the present invention, a laser diode having an active layer including quantum dots and a photo-resonator having a diffraction grating, the Bragg wavelength of the diffraction grating is determined to be within a predetermined energy width which is determined corresponding to the gain spectrum of the active layer, so that the line-width enhancement factor may be restricted within a required range that can be selected arbitrarily, and thus the chirping of the laser diode is reduced. Also, since the quantum dots are self-organized quantum dots, even if there is a detuning between the gain spectrum of the active layer and a Bragg wavelength of the diffraction grating due to a change of operating temperature of the laser diode, the Bragg wavelength remains within the range of the gain spectrum due to the broadening of the gain spectrum, and thus the laser diode remains oscillating.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.